This invention relates to two cycle engines and more particularly to an improved porting arrangement for such engines.
As is well known, two cycle engines are very popular because their ported nature makes them very simple. In addition, the firing of the cylinder for each revolution increases the specific output of the engine. However, there are a number of problems in connection with the utilization of ported engines.
One of the major problems deals with the fact that the intake cycle takes place at the same time and substantially overlaps the exhaust cycle. In fact, the intake cycle is utilized to purge the exhaust gases from the cylinder through a process that is commonly referred to as "scavenging."
However, when the scavenging is employed in an engine, there is a risk that the fresh air charge may also pass out of the exhaust port with some of the exhaust gases. This problem is particularly troublesome if fuel is also mixed with the exhausted mixture before it has had an opportunity to bum.
One popular type of scavenging system employed with two cycle engines is the Schnurle type. With Schnurle type scavenging, one or more scavenge ports are placed in proximity to the exhaust port. The flow of air into the combustion chamber from the scavenge ports is directed toward the opposing side of the cylinder wall for redirection upwardly and across the cylinder head. The charge then flows back downwardly to the exhaust port. This type of scavenging also uses, at times, an auxiliary scavenging port that is directly opposed to the exhaust port. Although this type of scavenging is very effective, there nevertheless is some concern that the fresh charge may pass out of the exhaust port.
A scavenging type of system has been proposed that employs a supplemental scavenge or tumble port that are disposed transversely to the main scavenge ports. This port introduces a tumble flow into the cylinder on the side facing away from the exhaust port. This permits the attainment of stratification and also improves or reduces the likelihood that fuel will pass out of the exhaust port. A construction of this type is shown in U.S. Pat. No. 5,671,703, issued Sep. 30, 1997 and assigned to the assignee hereof.
Although the system shown in that patent is very effective, there still seem to be ways to further improve performance. For example, it has been found that the utilization of tumble although helpful is not always desirable. There is, however, desire to at least redirect the scavenge flow from the main scavenge ports so that the charge is directed somewhat away from the side opposite to the exhaust port.
It has also been found that a scavenging system as described can very useful in achieving stratification if fuel is injected into the stream of air circulated from the supplemental scavenging port. In fact, in some instances it may be desirable to inject the fuel into the scavenging passage serving this port. Thus there are disclosed several improved arrangements in my copending application entitled Porting Arrangement for Two Cycle Engine, Ser. No. 09/038,388, filed concurrently herewith and assigned to the assignee hereof (Attorney Docket No. YAMAH4.554A) which are intended to improve stratification.
Although the arrangements shown in that concurrently filed application are particularly useful in achieving stratification, it is believed that further improvements are desirable. For example, the embodiments shown in that application all attempt to achieve stratification in a generally open combustion chamber.
The use of an open combustion chamber is desirable. However, with two-cycle engines it is very difficult to achieve stratification. The reason for this is that because of the fact that scavenging is taking place simultaneously with the exhaust of gases from the combustion chamber and the time period for achieving this is relatively small, mixing of the intake charge and fuel with the exhaust charge is difficult to avoid.
It is, therefore, a principal object of this invention to provide an improved combustion chamber and porting arrangement for an engine wherein fuel stratification can be achieved without sacrificing effective scavenging.
It is a still further object of this invention to provide an improved scavenging and stratification arrangement for a two-cycle internal combustion engine.